Week 2.2 - The menstrual cycle Flashcards
In general, What effects do oestrogen have on the gamete and the endometrium?
- Prepare the gamete for ovulation
- Prepare the endometrium for implantation
What is the function of the luteal phase?
-Hold the system in pause and maintain the endometrium until a signal is received that fertilisation has happened
In what rhythm is GnRH released?
-Pulsatile release every hour
What are the functions of FSH and LH in the female?
- Promote follicular development
- Stimulate follicular cells to produce steroid hormones and inhibin
- ovulation
What control do the gonadal hormones have upon HPG axis?
-Both positive and negative feedback
When does oestrogen have positive feedback effects?
-When it is high titres by itself ie the follicular phase
When does oestrogen exert negative feedback?
-When it is at moderate titres ie luteal phase
What is the dominant steroid hormone in the luteal phase?
-Progesterone
Describe the ovarian steroid hormone levels at the start of the cycle
-No ovarian hormone production
Which hormone encourages follicular development at the start of the cycle?
-FSH
Why is it that ovarian hormones drop at the end of the cycle? What effect does this have on the HPG axis?
- Follicular cells are dying and thus stop producing oestrogen/progesterone
- This resets the HPG axis as all feedback is removed as the levels are too low.
- GnRH can now work normally, causing slow release of FSH and LH
What effect does resetting the HPG axis have on the ovary?
-Recruitment of small group of follicles within ovary independent of extragonadal signals. This starts follicular development
Which cells of the ovary do FSH act on?
-Granulosa cells
When does oestrogen secretion become possible from the developing follicle? Why?
- When theca interna has developed -> has LH receptors
- Oestrogen production depends on interaction between theca and granulosa cells
What hormone, besides oestrogen, begins to be secreted from the developing follicle?
-Inhibin
What drives the formation of a dominant follicle? (2 things)
- Follicular oestrogen now at a level where it can exert a positive feedback control at the hypothalamus and pituitary
- Follicular inhibin levels rise which selectively inhibits FSH production by anterior production
What is the consequence on gonadotropin levels of inhibin?
-Only see a rise in LH, FSH secretion inhibited
There is a rapid rise in oestrogen and inhibin at approximately day 12 (when the follicle is fully developed), what effect does this have on gonadotropin hormones?
-LH surge
Why does progesterone production begin after LH surge?
-Granulosa cells become responsive to LH as prior to this they only had FSH receptors
What effect does oestrogen exhibit on the sensitivity of anterior pituitary?why does this cause the LH surge?
- High levels of oestrogen increases sensitivity of anterior pituitary to GnRH
- Early follicular phase -> Oestrogen low-> anterior pituitary not very sensitive to GnRH -> slow release of LH
- Late follicular phase ->oestrogen high-> sensitivity of the anterior pituitary is increased -> LH surge.
How does LH contribute to ovulation?
- Causes increase in collagenase activity -> follucle membrane breakdown -> oocyte released
- Causes PG increase -> contraction of ovarian wall to help extrusion
What happens to the primary oocyte in the follicle just before ovulation?
-Meiosis 1 completes and meiosis II starts
What happens to the follicle after ovulation?
-Remains in the ovary and become leutinised under the control of LH (forms corpus luteum)
What is the function of the corpus luteum?
- Secrete oestrogen and progesterone from androgens in large quantities
- Inhibin still pruduced to inhibit FSH
- Suppresses LH secretion by putting the axis into pause as further gamete development is suspended due to negative feedback of oestrogen and progesterone
What happens to the corpus luteum in the absence of LH?
-Spontaneous regression
What happens to the hormone profile after the corpus luteum has regressed?
-The axis is reset and everything is at basal levels
What happens to the corpus luteum if fertilisation occurs?
-Syncytiotrophoblast produces hCG which has luteinising effect and maintains the corpus luteum until the placenta is sufficiently developed to take over oestrogen and progesterone production to control HPG (approx 4 months)
What are the two phases in ovarian cycle?
- Follicular
- Luteal
What are the two phases in the uterine cycle?
- Proliferative
- Secretory
What causes the proliferation of the endometrium?
-Oestrogen produced from ovary
What effect does progesterone have on the endometrium?
-Causes it to enter the secretory cycle where it becomes very glandular to allow implantation
What effect does oestrogen have in the follicular/proliferative phase?
- Increases mobility of uterine tube
- Thickening of endometrium
- Growth and motility of myometrium
- Thin alkaline cervical mucus
- Vaginal changes
- Changes in skin, hair and metabolism
What effect does progesterone have in the luteal/secretory phase?
- Further thickening of the endometrium to become glandular
- Thickening of myometrium but reduces motility
- Thick acid cervical mucus
- Changes in mammary tissue
- Increases in body temperature
- Metabolic and electrolyte changes
Why in the luteal phase, does the cervical mucus become thick and acidic?
- Limit polyspermy
- Prevent infection
What is the normal limit for menstrual cycle? How can it vary?
- 21-35 days
- Luteal phase always 14 days (+/- 2 days) it is the follicular phase which varies
What is the ovarian cycle?
-Preparation of the gamete for ovulation
Describe the relationship between theca interna and granulosa cells in producing oestrogen
- Theca interna has receptors for LH which causes the production of androgens (androstenedione) which are precursors of eostrogen
- Granulosa cells have the enzyme aromatase which use the androgens secreted by theca interna to oestrogens
